Field of the Invention
The present invention relates to guide vane assemblies for compressors, in particular high-pressure compressors, designed for turbomachines, such as turbojet engines for aircraft.
Description of the Related Art
Generally, compressors for turbojet engines comprise a plurality of successive stages aligned along the longitudinal axis of the engine and made up alternately of movable stages forming the rotor of the compressor, the vane assemblies of which accelerate the gas flow by deflecting it relative to said axis, and fixed stages forming the stator, the vane assemblies of which partly transform the speed of the flow under pressure and guide said flow towards the following movable stage.
The final stage or stages of the stator of the high-pressure compressor are sectorised guide vane assemblies which principally, after successive assembly of the sectors one after the other in an external housing casing, form two concentric shell rings, one outer and one inner, between which are arranged the vanes of the vane assembly through which the gas flow, at that stage the primary gas flow, passes in a dual-flow turboshaft engine. The outer shell ring is provided with means of attachment such as the peripheral engagement rims (of curved form), referred to as front and rear according to the direction of the flow, for connecting the external casing of the stator of the compressor, whereas the inner shell ring has abradable devices on its outside connected with sealing devices for the rotor concerned.
In a turbojet engine, guide vane assemblies are parts that work statically (aerodynamic forces and mechanical forces passing through the external casing) as well as dynamically (for example, in relation to significant vibrations during transitory engine operation phases), and their dimensions are therefore defined in advance using a Haig curve which can determine their mechanical strength and resistance to fatigue.
Using this curve, the maximum admissible dynamic stress at a point on the part in question is therefore determined for a given static stress at that point. In addition, it is known from experience that the greatest possible maximum dynamic stress is required so as to be able to tolerate greater vibratory responses on the engine.
In the case of conventional fixed guide vane assemblies, the area of maximum static stress and the area of dynamic stress are located at the same place on the guide vane assembly, that is, at the rear of the cylindrical outer shell ring formed by the assembled sectors. Therefore, the admissible dynamic stress is greatly reduced because the maximum static stress is located at the same place, which limits the operating possibilities of the guide vane assembly and its resistance to fatigue, in particular at sustained vibratory engine speeds.
Patent FR 2 945 331 by the applicant discloses a solution for optimising dynamic stress, said solution consisting of drilling a horseshoe-shaped hole in the cylindrical wall of the upper shell ring, between the rear rim and the trailing edge of at least some of the vanes which are welded to the wall, so as to make the shell ring more “flexible” locally. This allows the static stresses in the blend radius of the curved rear rim to be considerably reduced so as to increase the maximum dynamic stress and push up the fatigue limit of the guide vane assembly in dynamic operation.
Although a solution of this type is permissible when there is enough space between the trailing edge of the vanes and the rear peripheral rim of the outer shell ring to drill the hole there, if this space is insufficient, the solution is not possible as it would require the hole to also be drilled through the rear rim in order to pass through the wall of the shell ring. Such a solution would weaken the guide vane assembly too much and would therefore be unsuitable for this type of guide vane assembly.